It works for me If i change in Firefox Preferences -> Home -> Homepage and new Windows select blank or custom and enter the page you want.
Another way is to type about:config in address bar, search for mxlinux.org and change the homepage variable, but I recommend using the GUI method above.

try this:
visit about:config
search "browser.startup.homepage" (noquotes)
double-click that row item, then type or paste your desired homepage URLstring into the popup dialog.
If you want the "firefox homepage", set browser.startup.homepage=about:home
The change should take effect immediately. Open a new tab to test the result.

There's a separate startup.homepage_welcome_url prefkey, but I reckon you won't need to change it.
(By default it's set to load the "let's trick the weak-minded into believing they need to signup for a 'firefox account' " page)